As lasers are finding more and more applications in research, medicine, industry and other various fields, new or improved laser sources are constantly needed. A common goal is to design higher power sources, while maintaining or improving other characteristics, especially beam quality. A way of achieving such laser sources is by optical pumping of a solid-state material with laser diodes. These offer a much narrower emission spectrum than previously used flash lamps, not mentioning higher efficiency and longer lifetimes among many advantages. This results in optimized absorption of the pump light at s wavelength close to the medium's absorption peak, often allowing the pump light to be absorbed in a single pass through the laser medium. However, many of these materials exhibit different absorption coefficients depending on the pump light's polarization. Such materials include Nd:YVO4, Nd:GdVO4, Nd:YLF, NdYAlO3 Nd:LSB and Nd:YAP. One common combination is a Neodymium-doped Vandadate crystal pumped by laser diodes emitting at Neodymium's absorption peaks, around 808 nm or 880 nm. The latter may be preferred to the first for reducing the quantum defect, effectively limiting the crystal's thermal load. Therefore lower crystal temperature and reduces bulging of end surfaces are achieved, leading to lower and less aberrated thermal lensing. Conversely, higher pump power may be applied to the crystal with regard to 808 nm pumping. Such direct pumping of the upper laser level of Vanadate, along with experimental comparison with 808 nm pumping, is described in Dudley et al “Direct 880 nm Diode-pumping of Vanadate Lasers”, Cleo 2002.
Then, the crystal's doping concentration in active ions is selected to achieve the desired absorption length. In many cases, a short absorption length is desired to localize the gain region to a small volume, in order to optimize pump/laser mode matching. However, scaling such pumping schemes to higher power requires increasing the pump light's absorption length in order to spread the heat load in a larger volume. A common way of achieving this is to lower the crystal's doping concentration, keeping the diodes emitting at the same wavelength. Choosing the right doping allows to tailor the absorption to specific needs or setups. This technique and various embodiments are described in Cheng et al., “Lasers with low doped gain medium”, U.S. Pat. No. 6,185,235.
Using very long crystals requires spreading the absorption on its whole length, therefore reducing the doping to a very low value. However the available crystal growing technology cannot achieve very low doping while maintaining acceptable accuracy on the concentration in active ions. For example, Neodymium Vanadate crystals' availability is currently limited to about 0.1% atm. Nd doping, +/−50% relative accuracy. The use of such crystals is therefore prohibited when repeatable performance and characteristics are desired, without individually selecting each crystal. One is therefore limited in the choice of pumping schemes and crystals, so that the desired absorption length can be achieved with low-doped materials readily available. Thus, in order to use long media and spread the absorption along its length, a solution is needed to achieve very low absorption in materials of available doping concentration.
Another limit to optimized pump absorption arises from the polarization dependence of absorption in certain laser materials, as previously mentioned. Neodymium Vanadate (Nd:YVO4) will be used to illustrate the different concepts and physical effects, as it is at present a very attractive and widely used laser material, exhibiting a strong polarization dependence of absorption. It should be noted that this specific laser crystal is solely stated for illustration purposes, and does not restrict in any way the scope of the invention.
Nd:YVO4 exhibits a great difference in absorptions on its a and c axes around its absorption peaks and usual pump wavelengths of 808 and 880 nm (αc=3.7 αa at 808 nm, and αc=3 αa at 880 nm). Many high power end-pumped schemes make use of fiber coupling or other devices to deliver the pump light from the diodes to the crystal. Most of these delivery systems do not maintain the original linear polarization of the diode's emission, leading to unpolarized or partially polarized output. Therefore the absorption length of this pump light will depend on how it's split between the two polarizations along the crystal's a and c axes. Furthermore, the proportion of pump light polarized on each axis can depend on environmental factors—rotation or twisting of the fiber—that cannot be easily controlled. The pump absorption length will then depend on these factors, leading to variable laser output characteristics. One way of circumventing that problem is to depolarize the pump light before or after the pump delivery system, leading to unpolarized light. Petersen, U.S. Pat. No. 5,999,544 describes such a system used in conjunction with fiber-coupled diode bars. Although the use of such depolarizer guaranties insensitivity to environmental influences at the cost of minimum added complexity, the absorption curve and length will be identical to that of unpolarized light.
One should notice however that the absorption of unpolarized light by a medium exhibiting different absorption coefficients along two or more of its axes doesn't allow the same optimization and ultimate performance as with polarized light. In the case of Vanadate, most of the pump light polarized along the crystal's c-axis is absorbed within a short distance from the entrance face, whereas it takes a much longer distance for the light polarized on the a-axis to be absorbed. Then, for the same total absorbed power, polarized pumping provides lower absorbed pump power density close to the entrance face, spreading the absorption on the whole length of the crystal. On the contrary, unpolarized pumping creates a higher thermal load close to the entrance face, leading to bulging of the crystal's surface, higher temperature, and effectively higher thermal lensing and aberrations.
One solution to reducing the effects of such strong absorption close to the entrance surface of the pump light is described in Marshall, U.S. Pat. No. 6,144,484. Undoped end-caps are diffusion bonded to the crystal's ends to act as a thermal reservoir, thus reducing the pump induced bulging of the crystal and keeping the crystal's pumped volume at a lower temperature than with free-standing ends. This results in reduced and less aberrated thermal lensing, ultimately increasing the maximum applicable pump power while maintaining beam quality. However this is just an improvement to classic end-pumping, allowing slightly higher power to be applied to the crystal, but not reducing the high local thermal load itself. Furthermore the cost and the limited availability of such crystals with diffusion-bonded end-caps limits their use in a product. Is it therefore desirable to use polarized rather than unpolarized light to optimize the pumping scheme and finally the laser's overall performance.
However, many end-pumped systems make use of optical fibers, fiber bundles or other homogenizing devices for pump light delivery, providing spatially homogenized light distribution to the crystal, but without the conservation of the diode's linear polarization. One technique for maintaining this polarization or achieving low depolarization is to use very short single fibers, maintained straight in a fixed position. Although this technique provides close to linearly polarized light, spatial homogenization isn't as good as with longer fibers. There is therefore a tradeoff between beam quality and polarization conservation. Furthermore, the use of such polarization-maintaining technique cannot always be implemented or is too impractical for certain configurations, not allowing separation of the pump source from the laser cavity.
Another technique for delivering polarized light to the laser medium, even after partial or total depolarization of the pump light through a delivery or homogenizing device, is presented in Maag et al., U.S. Pat. No. 6,137,820. The partially or totally unpolarized pump beam is split by a polarizing element in two beams of orthogonal linear polarizations. One of the two beams passes through a polarization-rotating element (e.g. a half-wave plate oriented at 45° of the incident polarization) that rotates the polarization by an angle of 90°, making the second beam linearly polarized parallel to the first. Then these two beams pump the laser crystal either independently from both sides, or superimposed on one face of the crystal. The crystal is therefore pumped on one single polarization, either on the strong or the weak absorption axis, depending on the desired configuration. Although such system provides linearly polarized pump light, it is split in two beams that need to both overlap with the laser mode in the crystal. Thus, the choice of pumping configurations is limited by this requirement. Furthermore, additional components are needed (polarizer, wave-plate, lenses, etc. . . . ), which increases the complexity of the system, thus forbidding its use in most applications and products.
Therefore there is a need for a technique that allows pumping a material exhibiting polarization-dependent absorption with unpolarized or partially polarized light, while keeping the benefits of pumping with linearly polarized light.
There is a further need for a technique that allows to increase the absorption length beyond what is achievable with low-doped crystals and regular laser diodes emitting around the absorption peaks, effectively allowing higher pump power to be applied compared to low-doped crystals pumped at the peak wavelengths.